ABSTRACT

SONYC—Substellar Objects in Nearby Young Clusters—is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. For the ~1 Myr old ρ Ophiuchi cluster, in our earlier paper we reported deep, wide-field optical and near-infrared imaging using Subaru, combined with Two Micron All Sky Survey and Spitzer photometry, as well as follow-up spectroscopy confirming three likely cluster members, including a new brown dwarf with a mass close to the deuterium-burning limit. Here we present the results of extensive new spectroscopy targeting a total of ~100 candidates in ρ Oph, with Fiber Multi Object Spectrograph at the Subaru Telescope and SINFONI at the ESO's Very Large Telescope. We identify 19 objects with effective temperatures at or below 3200 K, eight of which are newly identified very low mass probable members of ρ Oph. Among these eight, six objects have T eff <= 3000 K, confirming their likely substellar nature. These six new brown dwarfs comprise one-fifth of the known substellar population in ρ Oph. We estimate that the number of missing substellar objects in our survey area is ~15, down to 0.003-0.03 M sun and for AV = 0-15. The upper limit on the low-mass star to brown dwarf ratio in ρ Oph is 5.1 ± 1.4, while the disk fractions are ~40% and ~60% for stars and brown dwarfs, respectively. Both results are in line with those for other nearby star-forming regions.

Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (program 385.C-0450).